Polyaniline (PAn) films were deposited on an expanded graphite electrode by potentiostatic method. The electrochemical synthesis conditions of PAn were discussed in detail in this paper. The results showed that PAn with high electrochemical activity was obtained by electropolymerization of aniline in a solution containing 0.2 mol/L aniline and 0.3 mol/L sulfuric acid at 0.8 V. A three-electrode system composed of PAn/expanded graphite or magnesium which served as the working electrode, platinum sheet as the counter electrode and saturated calomel electrode (SCE) as the reference electrode was used for testing of the polarization curves of PAn and magnesium in 0.1 mol/L sodium nitrite, magnesium chloride, magnesium nitrate, zinc nitrate and phosphate buffer. The results indicated that the type of electrolyte had little effect on the stability of PAn, however, magnesium presented the best stability in 0.1 mol/L sodium nitrite. Mg│NaNO₂│PAn battery was fabricated in which PAn and magnesium were taken as cathode and anode, respectively. This battery was discharged by a galvanostatic method, and the current density was set as 30 mA•g^－1 in the discharge process. Compared with other electrolytes, this battery showed higher open circuit potential and specific discharge capacity in 0.1 mol/L NaNO₂.